One of the driving methods of a light-emitting device is a time gradation method which controls a length in which a pixel emits light in one frame period using a binary voltages that digital video signals (hereinafter referred to video data) have to display gradation. Specifically, in the case where display is performed by a time gradation method, one frame period is divided into a plurality of subframe periods. Then, in accordance with a value of one bit (hereinafter referred to as a video bit) among a plurality of video data bits, a pixel is to be a state of light emission or non-light emission in each subframe period. The length of light emission and non-light emission is different between video bits, and a most significant video bit is the longest and a least significant video bit is the shortest.
One example of a conventional time gradation display device is described with reference to FIG. 1. A pixel portion 107 is arranged centrally. In the pixel portion, a current supply line 106 for supplying a current to an EL element (means a light-emitting element using an electroluminescence material) is arranged as well as a source signal line and a gate signal line. Above the pixel portion, a source driver circuit 101 for controlling the source signal line is arranged. The source driver circuit 101 has a first shift register circuit 103, a first latch circuit 104, a second latch circuit 105 and the like. On the left of the pixel portion, a gate driver circuit 102 for controlling the gate signal line is arranged.
As for the source driver circuit 101, a configuration as shown in FIG. 2 is provided, and a shift register circuit (SR) 201, a first latch circuit (LAT1) 202, a second latch circuit (LAT2) 203 and the like are provided. Note that although not shown in FIG. 1 and FIG. 2, a buffer circuit, a level shifter circuit or the like may be arranged if necessary.
An operation is briefly described with reference to FIGS. 1 and 2. First, a clock signal (referred to as S-CLK and S-CLKb in FIG. 2) and a start pulse (referred to as S-SP in FIG. 2) are inputted to the first shift register circuit 103 (referred to as SR in FIG. 2), and sampling pulses are outputted sequentially. Subsequently, the sampling pulses are inputted to the first latch circuit 104 (referred to as LAT1 in FIG. 2), and video data (referred to as Digital Data in FIG. 2) similarly inputted to the first latch circuit 104 are held. In the first latch circuit 104, in one horizontal period, when holding of each digital video signal for one bit by each latch is completed, the digital video signals held in the first latch circuit 104 are transferred to the second latch circuit 105 (referred to as LAT2 in FIG. 2) all at once in accordance with an input of a latch signal (referred to as Latch Pulse in FIG. 2) during a fly-back period.
On the other hand, in the gate driver circuit 102, a gate side clock signal (G-CLK) and a gate side start pulse (G-SP) are inputted to a second shift register circuit 108. The second shift register circuit 108 outputs pulses sequentially based on these input signals and through a buffer (not shown) or the like, outputted as a gate signal line selection pulse to select a gate signal line sequentially.
Data transferred to the second latch circuit 105 in the source driver circuit 101 is written in pixels of a column selected by the gate signal line selection pulse.
Subsequently, the drive of the pixel portion 107 is described. FIG. 3 shows a part of the pixel portion 107 of FIG. 1. FIG. 3 (A) shows a matrix of 3×2 pixels. A portion surrounded by a doted line frame 300 is one pixel and its enlarged view is shown in FIG. 3 (B). Reference numeral 301 is a TFT (hereinafter referred to as a switching TFT) which functions as a switching element when a signal is written in the pixel in FIG. 3 (B).
Either polarity of an N channel type or a P channel type may be used for the switching TFT 301. Reference numeral 302 is a TFT (referred to as an EL driver TFT) which functions as an element (a current control element) for controlling a current supplied to an EL element 303. In the case where a P channel type is used for the EL driving TFT 302, the EL driving TFT 302 is arranged between an anode 309 of the EL element 303 and a current supply line 307. As another structure method, an N channel type can be used for the EL driving TFT 302 and the EL driving TFT 302 can be arranged between a cathode 310 of the EL element 303 and the current supply line 307 as well. However, since a grounded source is preferable for a TFT operation, the restriction in manufacture of the EL element 303 or the like, a method is common and often adopted in which a P channel type is used for the EL driving TFT 302 and the EL driving TFT 302 is arranged between the anode 309 of the EL element 303 and the current supply line 307.
A storage capacitor 304 is to hold a signal (voltage) inputted from a source signal line 306. Although one terminal of the storage capacitor 304 in FIG. 3 (B) is connected to the current supply line 307, a dedicated wire may also be used. A gate electrode of the switching TFT 301 is connected to a gate signal line 305 and a source region thereof is connected to the source signal line 306.
Next, with reference to FIG. 3, description is made on a circuit operation of an active matrix type light-emitting device. First, when the gate signal line 305 is selected, a voltage is applied to the gate electrode of the switching TFT 301 and the switching TFT 301 becomes a conductive state. Then, a signal (voltage) of the source signal line 306 is stored in the storage capacitor 304. Since the voltage of the storage capacitor 304 is a gate-source voltage VGS of the EL driving TFT 302, a current corresponding to the voltage of the storage capacitor 304 flows to the EL driving TFT 302 and the EL element 303. As a result, the EL element 303 emits light.
Luminance of the EL element 303, that is, the amount of the current flowing to the EL element 303 can be controlled by VGS of the EL driving TFT 302. VGS is equivalent to the voltage of the storage capacitor 304. That is, by controlling a signal (voltage) inputted to the source signal line 306, luminance of the EL element 303 is controlled. Finally, the gate signal line 305 is made into a non-selection state, the gate of the switching TFT 301 is closed, and the switching TFT 301 is made into a non-conductive state. At this time, a charge stored in the storage capacitor 304 is held. Therefore, VGS of the EL driving TFT 302 is held and a current corresponding to VGS continues flowing to the EL element 303 through the EL driving TFT 302.
The aforementioned drive of the EL element or the like has been reported in the following Non-Patent Document 1.
In a first display mode displaying images of 24 gradations by a time gradation display method, one frame period is divided into four subframe periods as shown in FIG. 4 (A) to display. In addition, in a second display mode displaying images of 2 gradations by a time gradation display method, one subframe period is included in one frame period as shown in FIG. 4 (B).
Display may be performed by changing a display control signal such that a whole surface is displayed by the first display mode in a certain frame period, using the subframe structure shown in FIG. 4 (A) whereas the whole surface is displayed by the second display mode in another frame period using the subframe structure shown in an indicator diagram 4 (B).
The aforementioned display driving methods are described in the following Patent Document 1 to Patent Document 3.
When displayed by using a time gradation method, a pseudo contour becomes a problem. In a pseudo contour, there are a moving image pseudo contour generated when a moving image is displayed and a still image pseudo contour generated when a still image is displayed. In frame periods appearing continuously, a moving image pseudo contour is generated by that a subframe period included in the preceding frame period and a subframe period included in a subsequent frame period are recognized as one sequential frame period by human eyes. That is, the moving image pseudo contour corresponds to an unnatural bright line or a dark line which is displayed on a pixel portion due to that the number of gradations which is different from the number of gradations which should be displayed in the normal frame period is recognized by human eyes.
Mechanism of generating a still image pseudo contour is similar to the case of the moving image pseudo contour. In the case of displaying a still image, a still image pseudo contour is generated by that a moving image seems to be displayed in a pixel in a vicinity of a boundary due to that visual points by humans are slightly moved left and right, and up and down in boundaries of regions in which the number of gradations are different from each other. That is, the still image pseudo contour corresponds to an unnatural bright line or a dark line which is generated so as to swing in the vicinity of a boundary due to that a moving image pseudo contour is generated in a pixel near the vicinity of a boundary of regions which have different numbers of gradations.
In order to prevent the aforementioned pseudo contour, it is effective to increase frame frequency, to further divide a subframe period into a plurality of numbers, or the like. The following Patent Document 4 describes a technique in which a subframe period is divided into a plurality of numbers and a period when a pixel emits light or a period when a pixel emits no light is prevented from continuing.
Although description is made using a P channel type for the EL driving TFT 302 in this specification, actually, a configuration using an N channel type may be used as well. In addition, VGS of the storage capacitor 304 is controlled using a binary voltage value by a time gradation method, and when the higher one of the binary is expressed by “1” and the lower one is expressed by “0”, in the case where a potential of the storage capacitor is “1”, a portion between the source and the drain of the EL driving TFT 302 becomes non-conductive and the EL element 303 emits no light whereas in the case where the potential of the storage capacitor is “0”, the portion between the source and drain of the EL driving TFT 302 becomes conductive and the EL element 303 emits light. In addition, in this specification, holding of “1” or “0” in the storage capacitor 304 is described as writing. Further, in a digital circuit which operates using a binary voltage value, the binary are expressed by “1” and “0”. Note that in signals specifying the logic of “1” and “0” in this specification, logic may be inverted. Here, when the potential of one electrode of the storage capacitor is “1”, the portion between the source and drain of the EL driving TFT 302 becomes conductive, while the potential of one electrode of the storage capacitor is “0”, the portion between the source and drain of the EL driving TFT 302 becomes conductive. In addition, in this specification, pixels including a gate signal line and the switching TFT 301 connected to a gate may be expressed as a row. Moreover, in this specification, in a time gradation method which displays for one frame period using a plurality of subframes, a period from a writing of video data to a pixel to the next writing of video data is defined as a subframe. Further, in this specification, among video bits, the most significant bit is described as a first bit, and a bit which is r bit (r is a natural number) lower than the most significant bit is described as a (1+r) bit.    Patent Document 1—Japanese Patent Laid-Open No. 2003-271099    Patent Document 2—Japanese Patent Laid-Open No. 2004-163774    Patent Document 3—Japanese Patent Laid-Open No. 2004-163777    Patent Document 4—Japanese Patent Laid-Open No. 2002-149113    Non-Patent Document 1—“Current Status and future of Light-Emitting Polymer Display Driven by Poly-Si TFT”, SID99 Digest: P372
In a display using a time gradation method, one frame period is divided into a plurality of subframes to display. Accordingly, as the number of the subframes in one frame period increases, the number of times for writing video data to a pixel and the operating quantity of a driver circuit of a display increase so that power consumption increases. On the other hand, in the case where the number of subframes is too small, the generation of the pseudo contour described in Background Art may be a problem.